Asthma is one of the most common chronic diseases and it is still on the rise with a prevalence estimated at 8- 9% of the U.S. population (~40 million) with an estimated cost of $50 billion/year in the US. New therapeutic approaches are needed that offer the potential for a long lasting cure, instead of merely suppressing chronic lung inflammation. Allergic asthma is the most common asthma endotype and is thought to be driven in large part by a type 2 airway immune response to inhaled allergens. Defining how memory CD4+ T helper type 2 (Th2) cells initiate an airway recall response to aeroallergens has the potential to offer new therapeutic approaches to treat allergic asthma. Recently, a new paradigm in memory T cell biology has emerged in which tissue-resident memory T cells (Trm) persisting in non-lymphoid tissue are critical for initiating antigen-specific recall responses in peripheral tissue. Trm are a unique subset of memory T cells that are anatomically positioned and transcriptionally programmed to initiate the tissue amnestic response to antigen. During periods of disease quiescence, approximately 5-10% of effector Th2 cells driving allergic asthma give rise to tissue- resident memory Th2 cells (Th2-Trm) that are retained in the lung and are thus poised to respond upon allergen re-exposure. While Th2-Trm have been described in a murine model of asthma, how they promote allergic inflammation is unclear. The objective of this proposal is to define the mechanisms whereby Th2-Trm persisting in the lung orchestrates a recall response to inhaled allergens. Our central hypothesis is that Th2- Trm ignite allergic airway inflammation via a rapid and enhanced response to cognate antigen in the airway and the ability to recruit circulating Th2 cells (Th2-Tcr) to the sites of antigen presentation in the lung. Mechanistically, we hypothesize that Th2-Trm co-localize with DCs expressing the Th2 cell-attracting chemokine CCL17 and after allergen re-challenge rapidly produce type 2 cytokines that initiate allergic inflammation and markedly enhance DC expression of CCL17. This increased CCL17 expression recruits Th2- Tcr cells from the blood to sites of antigen presentation where Th2-Tcr receive a ?second touch? from cognate antigen loaded and activated DCs and become fully competent to amplify allergic inflammation. We propose to use innovative experimental systems to define the function of Th2-Trm, including parabiosis, a novel CCR4 ligand ?CCL17 and CCL22? dual reporter mouse, and single cell RNA-seq analysis of murine lung and human airway mucosal CD4+ T cells obtained via bronchial brushing. Specifically, we propose: (1) To define the function of lung-resident Th2-Trm in regulating recurrent allergic airway inflammation in murine models and to define the transcriptional phenotype of murine and human lung Th2-Trm and Th2-Tcr; and (2) To define the role of CCL17 in regulating Th2-Trm and Th2-Tcr cell localization and function and to define the functional properties of CCL17-expressing lung DCs in promoting asthma. Defining the mechanisms regulating Th2-Trm function in the asthmatic airway has the potential to yield new therapeutic approaches for allergic asthma.